Manufacture of a Birefringent Liquid Crystal Component

ABSTRACT

Manufacture of a birefringent liquid crystal cell is performed as follows. A layer of isotropic material having an outer surface which is shaped with a surface relief structure and is provided with a liquid crystal alignment property is formed. A flexible sheet having an outer surface provided with a liquid crystal alignment property is formed. A curable birefringent liquid crystal material is applied to one or both of the layer of isotropic material and the flexible sheet. The flexible sheet is applied over the layer of isotropic material with the outer surfaces of the layer of isotropic material and the flexible sheet facing one another with the liquid crystal material therebetween, thereby to form a liquid crystal cell. The liquid crystal material is cured and the flexible sheet is removed from the liquid crystal cell.

The present application is a divisional application of U.S. patentapplication Ser. No. 12/441,480 filed on Mar. 16, 2009, which claims thebenefit from the priority of PCT application no. PCT/GB2007/004455,filed on Nov. 21, 2007, the disclosures of which are incorporated byreference herein in their entirety.

BACKGROUND

1. Field of Invention

The present invention relates to the manufacture of surface reliefbirefringent liquid crystal components. Such components are suitable foruse in a wide range of applications, including without limitationautostereoscopic 3D displays, brightness enhanced displays, for use inilluminating display devices, particularly transmissive andtransflective display systems and as a switching component for use inoptical networking.

2. Description of Related Art

Surface relief birefringent optical components are described for examplein WO-03/015424 and WO-2005/006056. A birefringent microlens array isformed from a surface relief interface between an isotropic material andan aligned birefringent liquid crystal material. Light of a first linearpolarisation state passing through the device sees a first refractiveindex step at the surface relief interface between the isotropicmaterial and the birefringent liquid crystal material, whereas light ofa second orthogonal linear polarisation state sees a second, differentrefractive index step at the interface. A birefringent component of thistype for a backlight is described for example in U.S. Pat. No.5,751,388. A backlight system of this type for providing emission ofpolarised light is described in US-2003/0,058,383, in which disclosure astructured birefringent optical component is arranged to deflect lightof a first polarisation and not deflect light of a second polarisation.

Birefringent liquid crystal components can be formed by means of aliquid crystal cell filling method as shown in FIG. 1. A rigid substrate2 such as a glass or polymer substrate, has an isotropic polymer layer 4formed on its surface by means of UV casting, embossing, thermal formingor other well known methods. The outer surface of the layer 4 is shapedwith a surface relief structure and has provided thereon an alignmentlayer 6, for example polyimide. The alignment layer 6 may be formed forexample by means of spin coating, printing or other known methods. Thealignment layer 6 is cured, and rubbed to produce a directional liquidcrystal alignment property. A second rigid substrate 8 with a secondalignment layer 10 forms a cell gap between the alignment layers 6,10which is capillary filled by birefringent liquid crystal material 12 asshown by arrow 14 typically at elevated temperature. The liquid crystalmaterial 12 may be a curable liquid crystal material. In this case,following filling, the material is cured, for example thermally, bylight or by electron beam radiation. Such materials allow highruggedness, and can enable a reduction in the thickness of devices.

Such a capillary filling process has a number of difficulties. Alenticular surface with an array of elongate cylindrical lenses is acommon surface relief structure. In this case, the capillary fill willoften take place along the length of the lenses. However, these lensesmay be susceptible to blockage, so that they do not fill uniformly,creating bubbles which degrade optical performance. In curable liquidcrystal materials, bubbles may contain oxygen which inhibits cure ofsome types of polymerisable liquid crystal material. This can causeregions of strain in the cured material, degrading alignment propertiesof the liquid crystal material near the bubble.

More even filling can be achieved by incorporating a larger spacer gapbetween the alignment layers 6 and 10. However, such an approachdisadvantageously uses more material, and so increases cost. Further,the uniformity of the thickness of additional material can be difficultto maintain, so the final device may not be flat, which may causenon-uniform optical output for example in an autostereoscopic displaysystem.

During filling, a vacuum can be used to avoid the formation of airbubbles. Vacuum equipment is disadvantageously expensive, and the highlevels of vacuum required for vacuum filling may not be compatible withthe lens polymer materials.

The device further requires two substrates 2 and 8. Such substratestypically have a thickness of 0.4 mm or greater. The overall thicknessof the display is thus large. To reduce thickness after fabrication, thepresent inventors have considered notionally removing the substrate 8from the cured liquid crystal material 12. However, this is problematic.Removal of the rigid substrate 8 is difficult. If the substrate 8 isformed from glass, it may be prone to cracking. The surface energies ofthe interface between the liquid crystal material 12 and the alignmentlayers 6, 10 may be similar, so that delamination may take placeunpredictably off either surface, therefore resulting in unreliabilityof delamination release. Further, the adhesion of the liquid crystalmaterial 12 to the alignment layer 6 is required to be as high aspossible, to maximise the endurance properties of the device. Highersurface energy may be achieved by addition of a wetting agent to theliquid crystal material 12. However, this may also increase the adhesionto the alignment layer 10, and thus reduce the reliability ofdelamination at the planar interface.

Further, the addition of alignment layer 10 adds cost to the processingmethod.

Further, the filling process can take some hours, particularly for alarge cell required for large displays or for motherglass processingmethods. Where the liquid crystal material 12 is a polymer liquidcrystal it may be liable to thermal cure prior to cure by for exampleultraviolet radiation. This means that such materials are difficult touse reliably in processes with prolonged process time. Premature curemay result in regions of non-uniform liquid crystal alignment andfilling errors.

Another difficulty is that when birefringent components such as shown inFIG. 1 are manufactured in motherglass form, it is difficult to cut themotherglass and separate the individual components after processing inmotherglass form. It is required to cut through the two differentsubstrates 8, 2 as well as through the cured polymer layer 4 and theliquid crystal material 12 without causing delamination of the polymermaterial 4 or cured liquid crystal material 12.

It would be desirable to provide a method of manufacture of a surfacerelief birefringent liquid crystal component in which at least some ofthese difficulties are alleviated.

SUMMARY

To this end, the present inventors have considered notionally theformation of a liquid crystal layer as a cured film by means of acoating process of liquid crystal material. The manufacture of uniformthickness birefringent optical films using liquid crystal in solvent isdescribed for example in U.S. Pat. No. 5,132,147; U.S. Pat. No.6,262,788; and Mock-Knoblauch, “Novel polymerisable liquid crystallineacrylates for the manufacturing of ultrathin optical films”, SID Digest2006. In the latter, a coating solution comprises a polymer liquidcrystalline material in a solvent solution. The solution is applied wetto the surface of a polymer film. The solvent is driven off and thematerial is exposed to UV light to cure the film.

FIG. 2 shows an apparatus notionally considered by the present inventorsto apply such a coating method to fabricate birefringent liquid crystalcomponents. Using this apparatus, a surface relief structure of apolymer layer 4 on a substrate 2, having an alignment layer 6, isovercoated by means of a coater such as a slot coater 19 filled withpolymerisable liquid crystal 12. Such an arrangement would be intendedto produce a film 21 of liquid crystal material in contact with air or agas such as nitrogen. The film is subsequently cured by means of a UVlight source 23. In principle this might alleviate some of thedifficulties associated with the use of the second substrate 8 in theknown manufacturing process described above with reference to FIG. 1.However, if such a process of coating of surface relief optical elementswith curable liquid crystals were in fact applied, then a number ofdifficulties are expected to arise as follows.

The typical dry film thickness of liquid crystal material in planardevices manufactured using the known coating methods is between 1 μm to10 μm, and requires the deposition of a wet thickness of liquid crystalmaterial of 10 μm to 30 μm. In contrast, the typical sag of the surfacerelief lenses used in autostereoscopic display is 15 μm to 60 μm, and sothe thickness of typical cured films of liquid crystal material wouldneed to be substantially more than that delivered by the known coatingmethods.

Further, the flow of the drying material on the component may notdeliver a flat surface due to differential drying properties across thewidth of the component.

Additionally, with such a notional technique, the film 21 of liquidcrystal material is aligned only by the alignment layer 6. This isproblematic. The interaction of the single alignment layer 6 with theliquid crystal material diminishes with increasing distance from thefilm 21. Thus alignment artifacts will appear in the liquid crystalmaterial, the alignment artifacts increasing with the thickness of thefilm 21. For relatively thick surface relief birefringent components, itis desirable that alignment is provided on both sides of the liquidcrystal material.

Furthermore, it is often desirable that there is a controlled twistbetween the alignment directions at the planar and surface reliefstructures on either side of the liquid crystal material. In structureswith a single alignment layer, a precise twist cannot be achieved asthere is no upper surface to define an alignment.

Further, the surface tension properties of coated microstructures canresult in the upper surface assuming a non-flat structure, withdifferent alignment properties at the cusps of the lenses compared tothe centre of the lenses. Such a structure will result in reducedoptical quality. The elements need to be maintained clean duringsubsequent handling. Therefore an additional protective cover may needto be added, further adding to the cost of the elements.

According to the present invention, there is provided a method ofmanufacture of a birefringent liquid crystal component, the methodcomprising:

-   -   forming (a) a layer of first material having an outer surface        which is shaped with a surface relief structure and is provided        with a liquid crystal alignment property, and (b) a flexible        sheet having an outer surface provided with a liquid crystal        alignment property;    -   applying a curable birefringent liquid crystal material to one        or both of the outer surface of the layer of first material and        the outer surface of the flexible sheet;    -   applying the flexible sheet over the layer of first material        with the outer surfaces of the layer of first material and the        flexible sheet facing one another with the liquid crystal        material therebetween, thereby to form a liquid crystal cell;    -   curing the liquid crystal material; and    -   removing the flexible sheet from the liquid crystal cell,        thereby to form a birefringent liquid crystal component.

This method of manufacture provides a number of advantages over theknown method described above with reference to FIG. 1 and over thenotional method described above with reference to FIG. 2.

Compared to the notional method described above with reference to FIG.2, the present method additionally involves the application of aflexible sheet having a liquid crystal alignment property, prior tocuring. Thus alignment is provided on both sides of the liquid crystalmaterial during the curing process. This provides a number ofadvantages. Most importantly, it reduces alignment artifacts. Similarly,it allows a reliable twist of alignment direction to take place withinthe birefringent liquid crystal material by applying the flexible sheetwith a twist between the alignment directions of the liquid crystalalignment properties of the flexible sheet and the layer of firstmaterial, which may be for example isotropic.

The alignment properties, such as pre-tilt angle and stability, of thelayer of first material and the flexible sheet need only be maintainedduring the polymerisation process rather than during the whole lifetimeof the component. The upper surface of the cured liquid crystal materialafter cure can be maintained flat across the area of the device,improving optical quality.

The application of the liquid crystal material is straightforward toperform. It may be applied to either or both of the flexible sheet andthe layer of isotropic material. It is possible to use a coatingtechnique of the known type described above but this is not essential,it being a particular advantage that the liquid crystal material may beapplied more quickly and with less precision than in the known coatingtechniques, due to the use of the flexible sheet forming an upperboundary for the liquid crystal material. For example, the liquidcrystal material may be deposited over a limited area and forced acrossthe component by the flexible sheet.

Furthermore, as a result of removing the flexible sheet after curing,these advantages are achieved in the resultant birefringent liquidcrystal component which does not have a substrate over the liquidcrystal material on the opposite side from the layer of first materialproviding the surface relief structure. Thus the resultant componentdoes not suffer from the disadvantage of the known method describedabove with reference to FIG. 1 that the component is very thick.

Furthermore, as compared to use of two rigid substrates, the flexiblesheet is very easy to remove, for example by peeling. Due to itsflexibility, the surface energy of the flexible sheet means that it canconveniently be delaminated from the liquid crystal cell after curing ofthe liquid crystal material.

Other advantages over the known method described above with reference toFIG. 1 are as follows.

The liquid crystal material does not need to be capillary filled and canbe applied in a very short timescale over large areas compared tocapillary or vacuum filling. The components can thus be made withoutbeing prone to premature cure. Further, as the flexible sheet covers theliquid crystal material during curing, the component does not have to bevacuum filled or cured in a nitrogen blanket, thus reducing cost andcomplexity of equipment.

The flexible sheet can have a low thickness so that the cost of materialcan be minimised.

When it is desired to use a liquid crystal material curable byelectromagnetic radiation, the flexible sheet may be transparent to thatelectromagnetic radiation, and the step of curing the liquid crystalmaterial is performed by applying said electromagnetic radiation throughsaid flexible sheet.

The method may conveniently be applied to components made in motherglassform. In this case, plural birefringent liquid crystal components aremade together in motherglass form and the method further comprisescutting out the individual birefringent liquid crystal components aftercuring of the liquid crystal material, for example by means of lasercutting. Alternatively, the cured structure can be scribed and brokenusing standard glass cutting techniques.

The material of the flexible sheet may be selected so that the liquidcrystal alignment property is provided by an intrinsic property of thematerial. The alignment property may be further enhanced by means of atreatment such as rubbing prior to application. In this case, theflexible sheet does therefore not require the formation of separatecoated and processed alignment layers, and is thus cheaper tomanufacture than a coated glass substrate.

Lenticular displays in which a lens array is attached to the surface ofa spatial light modulator are well known. Typically a lens in air isattached in alignment to the display. However, the reflectivity of alens surface in air is 4-5% using standard materials. Such lensesproduce high levels of frontal reflections, being curved surfaces, andreduce front of screen contrast in brightly lit environments. Highscreen surface visibility reduces the ability to present large amountsof image depth, as it provides a visual conflict with stereoscopic depthcues. Such lenses use materials and surfaces which are expensive ordifficult to coat with standard anti-reflection layers. Further, suchlenses in air exhibit total internal reflection artifacts. It would bedesirable to reduce the Fresnel reflections and total internalreflection artifacts from such lenses while providing surfaces suitablefor low cost anti-reflection coatings.

In prior art lenticular display systems in which a lenticular screen isattached to the output of a spatial light modulator, there arenotionally two arrangements for the lenticular screen comprising aplanar surface and a surface relief surface. In a first arrangementlight passes from the spatial light modulator, through the planarsurface, through the lenticular screen material and is output throughthe surface relief surface. Such a lens suffers from reflectivity at thelens surface which diffuses an ambient light source, providingvisibility of the lens surface, and thus degrading the 3D image quality.In a second arrangement, light is output from the spatial lightmodulator into an air gap, is incident on the surface relief surface,passes through the lenticular screen material and is output through theplanar surface. Such an arrangement suffers from substantially the samereflectivity artifact at the curved lens surface as for the firstarrangement, but also typically exhibits noticeable total internalreflection artifacts in which ambient light sources are reflected by theinternal surface of the surface relief structure. This total internalreflection provides greater levels of surface visibility and thusdegrades device performance.

Lenses which use low refractive index materials in contact with a lenssurface are known. However, in order to generate adequate refractiveindex step, it is often reported that it is necessary to use materialssuch as fluoropolymers as one of the materials. Fluoropolymers areexpensive and can be difficult to adhere to the panel. Alternatively,low refractive index materials such as silicone oils can be used, butthese require sealing to prevent leakage. Such lenses have reducedreflectivity, because of the reduced index step at the surface, butreflectivity of the lens surface is present in both polarisation states.

The birefringent optical elements in this invention can be used innon-switching 3D displays with reduced Fresnel reflection and totalinternal reflection artefacts. Such displays advantageously have verylow surface visibility and can be used in brightly lit environmentswithout significant degradation of front-of-screen contrast. Suchdisplays are able to demonstrate increased out-of-screen depth ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

To allow better understanding, an embodiment of the present inventionwill now be described by way of non-limitative example with reference tothe accompanying drawings, in which:

FIG. 1 is a side view of surface relief birefringent liquid crystalcomponent during a prior art filling method;

FIG. 2 is a side view of a notional coating apparatus;

FIG. 3 a is a side view of an apparatus for manufacturing a surfacerelief birefringent liquid crystal component;

FIG. 3 b is a flow chart of a method of manufacture of a surface reliefbirefringent liquid crystal component;

FIG. 3 c is a schematic view of alignment orientations on surfaces asurface relief birefringent liquid crystal component;

FIG. 3 d is a side view of a further manufacturing apparatus;

FIG. 3 e is a side view of a further manufacturing apparatus;

FIG. 3 f shows the use of spacer layer to increase LC thickness during acoating process;

FIG. 3 g shows the structure of a tool;

FIG. 3 h shows in planar view the cutting of a motherglass substrate;

FIG. 4 is a side view of a backlight and display using a birefringentdiffuser;

FIG. 5 is a side view of a birefringent diffuser;

FIG. 6 is a side view of an alternative backlight arrangement using asurface relief birefringent liquid crystal component;

FIG. 7 is a side view of a projection apparatus incorporating surfacerelief birefringent liquid crystal components;

FIG. 8 is a side view of a beamsplitting optical element; and

FIG. 9 is a side view of a dual layer focussing device.

FIGS. 10 a and 10 b show in planar view and side view, respectively, thealignment directions in a lens of the invention;

FIG. 11 a shows a double lens;

FIG. 11 b shows an alignment mechanism for a double lens;

FIG. 12 shows a 3D only display using the lenses of the presentinvention; and

FIGS. 13 a and 13 b are side views of a further embodiment.

DETAILED DESCRIPTION

There will now be described with reference to FIG. 3 b a method ofmanufacture of a surface relief birefringent liquid crystal componentwhich may be performed using the apparatus shown in FIG. 3 a. The stepsshown in FIG. 3 b can be carried out in any order, not limited to theorder in which they are described below.

In step 30, a rigid substrate 2, which may be made of glass or apolymer, has a surface relief layer 4 formed on its surface. The surfacerelief layer 4 comprises a first material which in this example isisotropic and a polymer. The outer surface of the surface relief layer 4(uppermost in FIG. 3 a) is shaped with a surface relief structure, inthis case comprising an array of cylindrical lens surfaces. Thesubstrate 2 may alternatively be semi-rigid or flexible, for examplewhen flatness or dimensional stability of the birefringent liquidcrystal component is not critical. The substrate 2 may in some cases beof the same material as the isotropic polymer material 4.

In step 32, the surface relief structure of the surface relief layer 4is treated, for example by washing, and/or by UV ozone treatment toremove surface contamination.

An alignment layer 6 is coated in step 34 onto the surface of the lens,and cured as well known in the art, followed by a rubbing process step36. The alignment layer 6 provides the surface relief structure of thesurface relief layer 4 with a liquid crystal alignment property.Alternatively, step 36 can comprise a photoalignment step.

The substrate is then heated in step 38 in the apparatus shown in FIG. 3a, for example by means of a heater pad 26, by means of blowing hot airacross the surface of the device, or by placing the device in an oven atthe desired temperature, say 90 degrees Celsius. The process temperatureis typically above the crystalline to nematic transition temperature ofthe polymerisable liquid crystal material 12 which is described furtherbelow, but low enough to minimise thermal cure of the material. If amaterial which supercools is used, the processing temperature mayadvantageously be below the crystalline to nematic transitiontemperature. With suitable materials this can include room temperature.

In step 40, a UV curable liquid crystal material 12 is mixed with aphotoinitiator to improve cure by UV radiation. The liquid crystalmaterial 12 may be for example RM257 or RMM34c from Merck, or LC242,LC270 or LC1057 from BASF. The material may comprise mixtures ofmesogenic and non-mesogenic compounds. An additional nematic liquidcrystal material may be added to modify the refractive index andviscosity parameters, forming a liquid crystal gel material.

The liquid crystal material 12 may have a supercooling property so thatit maintains an aligned nematic state when the device is cooled belowthe crystalline to nematic transition temperature for an extended time.Such materials have a crystalline phase, melting to a nematic phase at atransition temperature. On cooling the nematic phase material, asupercooled state is achieved in which nematic properties of the liquidcrystal may be maintained. Supercooled materials advantageously have ahigher viscosity than materials above the crystalline to nematictransition temperature, and are thus less sensitive to flow during cure.Such materials advantageously allow the stress due to flow to beminimised during cure.

Possible photoinitiators for radical polymerization are for exampleIrgacure 651, Irgacure 184, Darocure 1173 or Darocure 4205 from CibaGeigy, or TPO-L from BASF when polymerizing by means of UV light. Thephotoinitiator concentration preferably comprises 0.01% to 10%, verypreferably 0.1% to 3%.

Also, a surfactant material such as Fluorad 171 from 3M Co., Zonyl FSNfrom DuPont or BYK361 from BASF may be incorporated into the liquidcrystal material 12 to improve surface wetting and adhesion of theliquid crystal material 12 to the alignment layer 6. The surfactantconcentration preferably comprises 0.01% to 1%. Further components mayinclude for example catalysts, stabilizers, chain-transfer agents, andco-reacting monomers.

Alternatively, the layer 4 may be formed from a material with a liquidcrystal alignment property and the layer 6 omitted. The surface of thelayer 4 may be rubbed, as well known in the LCD industry.

The materials may be mixed in step 40 in a solvent such asdichloromethane, methyl ethyl ketone or other well known low boilingpoint solvent. After mixing, the solvent is boiled off in step 42 in afume hood at room temperature for about twelve hours, leaving a driedresidue of mixed liquid crystal material 12. In step 44, the material isheated into the nematic phase and a vacuum applied to remove residualair bubbles from the mixture.

A flexible sheet 16 having an outer surface (lowermost in FIG. 3 d)provided with a liquid crystal alignment property is prepared asfollows. The flexible sheet 16 is sufficiently flexible to allow removalfrom the liquid crystal cell by peeling. As such it is more flexiblethan the rigid substrate 2. Alternatively, the flexible sheet 16 mayhave similar flexibility to the substrate 2. To optimise delaminationperformance, the adhesion of the material 12 to sheet 16 should be lessthan the adhesion at interfaces of material 12 to layer 6, layer 6 tomaterial 4 and material 4 to substrate 2.

Preferably the flexible sheet 16 is a film made of a polymer, forexample polyethyleneterephthalate (PET), polyvinylalcohol (PVA),polycarbonate (PC), triacetyl-cellulose (TAC), or any mixture thereof.Isotropic or birefringent substrates can be used. In case the substrateis not removed from the coated flexible sheet 16, preferably isotropicsubstrates are used. The thickness of the flexible sheet 16 may be inthe range, but not limited to, 50 μm to 300 μm.

Alternatively, the flexible sheet 16 may have a well definedbirefringence characteristic, and may be used as an additional waveplatein an optical structure, for example to increase the viewing angle of aseparate polarisation switching element (not shown).

In step 46, the stretch direction may be determined by examining theangle of the flexible sheet 16 for maximum extinction between crossedpolarisers and the flexible sheet 16 oriented with respect to thestretch direction.

The material of the flexible sheet 16 is chosen to provide a liquidcrystal alignment property intrinsically. Prior to application to thecell, in step 48 the flexible sheet 16 may be rubbed, as well known inthe LCD industry. The rubbing process may be aligned parallel to thestretch direction of the flexible sheet 16 if a birefringent film isused. The flexible sheet 16 may also be rubbed in other than the stretchdirection.

In step 50, the heated liquid crystal material 12 is applied to thesurface relief structure of the layer 4 by means of a dispenser 11 asshown in FIG. 3 a. The dispenser 11 may for example comprise a heatedfunnel such that the liquid crystal material 12 is applied in liquidform. Alternatively, the dispenser 11 may apply crystalline liquidcrystal material 12 which is heated above the nematic to crystallinetransition temperature in the apparatus. Advantageously, a singlecoating of liquid crystal material 12 is required. As shown in FIG. 3 a,in this embodiment the thickness of the liquid crystal material 12 isnot constrained and so the liquid crystal material 12 bulges above thelevel taken in the final component.

In step 52, the flexible sheet 16 is applied onto the surface of theliquid crystal material 12 by draping the flexible sheet 16 over thelayer 4 by means of an application bar 18 which moves in contact withthe exterior surface of the flexible sheet 16 over the flexible sheet16. In fact as apparent from FIG. 3 a, step 50 of applying the liquidcrystal material 12 and step 52 of applying the flexible sheet 16 areperformed simultaneously, the liquid crystal material 12 being appliedinto the gap between the layer 4 and the flexible sheet 16 in front ofthe application bar 18 as the flexible sheet 16 is draped on. Thus theapplication bar 18 squeezes the liquid crystal material 12 to acontrolled thickness governed by the height of the application bar 18.Thus the surface of the liquid crystal material 12 opposite from thesurface relief structure is planar.

The application bar 18 may be a member having a variety of forms. Theapplication bar 18 may have a circular cross section for example and mayoptionally have an outer coating 20, such as a rubber material.Alternatively the application bar 18 may have a non-circular crosssection, such as a rubber wiper. The application bar 18 may roll orslide as shown by arrows 22 and 24 respectively across the surface ofthe flexible sheet 16, thus trapping a layer of material 12 between theflexible sheet 16 and the alignment layer 6. The direction ofapplication is shown in FIG. 3 a as orthogonal to the geometric axis ofthe lenses of a lenticular array microstructure. However, in the case ofsuch elongate structures, it is generally preferable that theapplication direction is parallel to the geometric axis of the lenses sothat the material can flow uniformly down the channels without trappingair bubbles.

The orientation of the flexible sheet 16 with respect to the layer 4 isselected to provide a desired degree of twist between the alignmentdirections of the flexible sheet 16 and the layer 4. This adjusts thetwist angle of the liquid crystal director within the liquid crystalmaterial 12 after curing as described below. The alignment directionsmay for example be as shown in FIG. 3 c. The alignment direction on thesurface relief structure of the layer 4 may be parallel to the geometricaxis of the cylindrical lenses in a lenticular array and at an angle of135° to the aligning angle on the flexible sheet 16. Alternativealignment directions may be used depending on the requirements of theoptical architecture.

The technique for applying the liquid crystal material 12 in step 50 maybe varied from that described above, in general to apply the liquidcrystal material to either or both of the flexible sheet 16 and thelayer 4.

By way of example, FIG. 3 d shows a modification of step 50 in which theliquid crystal material 12 is coated onto the flexible sheet 16. Theliquid crystal material 12 may be applied to the flexible sheet 16 bymeans of a known coating technique and may be in nematic, supercoolednematic or crystalline states. Applying the flexible sheet 16 near tothe surface 6 may heat the material 12 into the nematic phase.Advantageously, the liquid crystal material 12 can be applied prior toapplication of the flexible sheet 16 in step 50, reducing complexity ofthe coating method. Advantageously, the required thickness of nematicliquid material can be set by determining the thickness of the layer onthe flexible sheet 16 prior to application to the layer 4.

As a further example FIG. 3 e, shows a modification of step 50 in whichthe liquid crystal material 12 is coated onto the layer 4 in advance ofstep 50 of applying the flexible sheet 16. The liquid crystal material12 may be coated by means of a known coating technique, for example bymeans of a bar coater, and may be in supercooled or crystalline stateprior to introduction into the heated coating apparatus. Advantageously,the liquid crystal material 12 can be distributed over the layer 4 priorto application of the flexible sheet 16 in step 50, reducing complexityof the coating method. The material 12 in FIG. 3 e does not need to beuniformly distributed, as the flexible sheet 16 will provide uniformityof thickness after coating.

In a further embodiment of the invention, the isotropic material 4 mayhave a structure 7 formed on its surface as shown in FIG. 3 f. Duringthe coating process, the structure 7 serves to provide a spacing for thefilm 16 and liquid crystal layer 17. The thickness of the film 16 may beadjusted to provide adequate thickness uniformity over the area betweenthe structures 7. Such a layer advantageously increases the liquidcrystal layer thickness so that an adequate amount of guiding may takeplace in the region of the lens cusps so as to allow a uniformpolarisation rotation across the whole area of the lens. FIG. 3 g showsthe structure of a tool 150 that may be used to form such a surface. Thesurface 152 has a lens structures corresponding to the desired lenssurface. The surface may have additional holes 154 formed in it. Whenthe replica is formed from the tool, the structures 7 are formed abovethe surface of the lens array. The holes 154 may be distributed in apattern or randomly. The holes may comprise linear structures.

In an alternative embodiment, spacer balls 9 can be mixed into the LCmaterial 12 as shown for example in FIG. 3 f. The pillars 7 and balls 9can be used together or as alternatives. The spacer may alternatively beformed in the sheet 16.

Alternatively, the liquid crystal material 12 may be applied in step 50to both the flexible sheet 16 and layer 4 prior to coating.

Advantageously, it may not be necessary to use the liquid crystalmaterial 12 under vacuum conditions reducing capital equipment cost andreducing the effect of contamination of the alignment for example bymaterials extracted from the surface relief layer 4.

After step 52, the device may be left to anneal residual disclinationsin step 54. The device can then be cooled in step 56 to a lowertemperature to increase the birefringence of the liquid crystal material12 and to increase its viscosity during cure or to tune the refractiveindex of the ordinary component to that of the substrate.

In step 58, the liquid crystal material 12 is cured into a solid film 17by means of actinic radiation such as electromagnetic (e.g. ultraviolet)radiation from a light source 28. The ultraviolet lamp may be filteredto remove wavelength components that may adversely affect the liquidcrystal material 12 through absorption. Advantageously, the flexiblesheet 16 enables the liquid crystal material 12 to be of the type, whosecure is inhibited by the presence of oxygen without the need for aninert gas blanket, thus reducing cost and complexity of the apparatus.

Following cure, a motherglass processing step 59 may take place as shownin FIG. 3 h in which the substrate 2 is cut and broken along lines 68 tocut out individual birefringent liquid crystal components. If thesubstrate 2 is glass, the cut may be by means of a scribe. The step 59may alternatively take place after steps 60 and 62. The phrasemotherglass processing is used generically to mean cutting of substrate2.

Subsequently, in a delamination step 60, the flexible sheet 16 isremoved from the plane surface of the layer 17 of liquid crystalmaterial 12. It is a particular advantage of that this removal is veryeasy as a result of the flexibility of the flexible sheet 16 providing arelatively low surface energy. For example the flexible sheet 16 maysimply be peeled off.

The flexible sheet 16 may be used as a protection film prior to deviceassembly to remove the need for a cleaning step after processing.Therefore, a further protective film is not required after processing.To allow the flexible sheet 16 to serve as a protective layer, thedelamination step 60 may be carried out at a later time after shippingand subsequent handling. Such a step advantageously avoids the need toclean the surfaces after fabrication and prior to device assembly,reducing cost and potential damage of the surface.

Optionally the substrate 2 could be removed by a delamination processperformed after step 58 (and before or after steps 60 and 62).

In alternative embodiments, the pressure and speed of the applicationbar 18 can be adjusted so that a gap is formed between the top of themicrostructure layer and the bottom of the flexible sheet 16, so as toallow for an increased thickness layer 17 of liquid crystal.

It may be possible to re-use the flexible sheet 16 after delamination,although typically this material would be re-processed or disposed of.

The flexible sheet 16 may further comprise a diffractive alignmentstructure so as to improve the alignment of the liquid crystal material12 at its surface. Optionally the flexible sheet 16 may be replaced by ametal film incorporating a diffractive alignment layer. In this case theliquid crystal material 12 can be cured from below through the lenses.

The pre-tilt of the alignment layer 6 may be reduced to minimise thedeflection of the liquid crystal material 12 towards the centre of thelens on the tilted microstructured surface. Reducing pretilt can thus beused to improve contrast of the lens device.

The component may be held on a heated vacuum chuck or other mechanism sothat the top surface is held flat, to avoid stress artifacts duringcooling.

The birefringent liquid crystal components manufactured by this methodmay be used for a variety of purposes. Some non-limitative examples willnow be given.

The component may be used as a birefringent microlens array for use in aswitchable autostereoscopic display apparatus, as described inWO-03/015424 and WO-2005/006056 for example.

The component may be used as a birefringent scattering element for usein a liquid crystal backlight apparatus for example as shown in FIG. 4.Such an apparatus may be used to recycle polarised light within thebacklight of a display, thus increasing device efficiency. Atransmissive or transflective display comprises a light source 100, awaveguide 102 and reflective film 104. Typically additional films areinserted including prismatic films 106 such as BEF from 3M, diffusers108 and reflective polariser films 110 such as DBEF from 3M. Thediffuser 108 can comprise a birefringent diffuser element such as shownin FIG. 5. Such a film may comprise a substrate 2, isotropic material 4and birefringent material 12. Light rays 122 of a first polarisation maysee an index match at the microstructured interface between thebirefringent and isotropic materials whereas light rays 124 may see anindex step so rays 122 are undeflected while rays 124 are deflected bythe backlight. Advantageously, the light rays that are diffusing arepassed through the polariser 110 and polariser 112 of the display whichfurther comprises a substrate 114, pixel layer 116, substrate 118 andoutput polariser 120. Light rays which are not diffused are reflected bythe polariser 110 into the backlight. The light is recirculated anddepolarised in the backlight, to be returned to the output of thedisplay. The birefringent diffuser advantageously allows light that willbe recirculated not to be diffused. Such an arrangement increases theefficiency of the reflection of the light rays 122 from the polariser110, and thus reduces the absorption of light in the polariser 112,increasing overall display efficiency.

The component may be used as a birefringent prismatic film for use in apolarised backlight.

The component may be used as a polarisation sensitive collimatingelement for a backlight is shown in FIG. 6. A light source 243, forexample an LED is positioned behind the aperture of a Fresnel lensaperture 241. The lens comprises a substrate 2, an isotropic layer 4 anda birefringent layer 12. Light of a first polarisation is substantiallycollimated by the lens 4, 241,12 whereas light of an orthogonalpolarisation is uncollimated. The collimated light is passed through apolarisation sensitive reflector 110 along rays 246. This light istransmitted through the input polariser of the LCD panel. Theuncollimated light is passed back into the backlight for recirculation.Such an arrangement provides improved diffusion of the recirculatinglight, whereas the collimated light from the backlight is passed throughthe panel, advantageously increasing display brightness characteristics.Such an arrangement further reduces the visibility of the LED lightsources. Alternatively, the collimated light may be reflected by thepolariser 110 and the uncollimated light may be transmitted at whichpoint it is diffused. As the incident light is on-axis, such anarrangement may increase the efficiency of reflection of light by thepolariser 110.

The component may be used as a polarisation sensitive projection screen,for example as shown in FIG. 7. An illuminated display device 200 isimaged by a projection lens 202. A polarisation switch 204 controls theoutput polarisation from the projection lens. Light of a firstpolarisation falls onto a projection screen 206 comprising abirefringent optical element of the present invention. The screen 206may be for example a Fresnel lens. The screen may direct light rays 212from an object 210 near the pupil of the projection lens to a region 214in front of the display. The screen 208 may comprise a birefringentdiffusing microstructure, which has no effect on the rays 212. In theorthogonal polarisation state, the screen 206 has no optical effect onthe rays 216 which after striking the screen 208 are diffused into a raybundle 218. Thus, a polarisation switch 204 can be used to control thedirectionality of a projection display. The diffuse mode can be used toprovide a wide viewing angle, whereas the directional mode can be usedto provide a high gain screen with high brightness from a limited rangeof angles. Such a screen may be appropriate for laser projectors forexample. The image 214 may be tracked in correspondence with themovement of an observer. Multiple images 214 may be produced frommultiple projection apertures 210, such that an autostereoscopic viewingfunction is enabled for example. In this case, the lens 206 is enabledwhile the diffuser 208 is disabled.

The component may be used as a polarising beamsplitters as shown in FIG.8. A prismatic microstructure surface 223 is formed on a substrate 220in a first polymer material 222 with a first isotropic refractive index.A birefringent material 224 is formed on the microstructure. An incidentlight ray 226 of polarisation state 228 sees a first index step at theinterface surface 223, and is deflected accordingly along light ray 230.Light of the orthogonal polarisation state 229 sees a different indexstep at the surface 223 and undergoes a different deflection along lightray 232. Such an element conveniently is low cost to manufacture and hasa flat structure compared to cube type polarisation beamsplitters. Suchan element may be used in position encoders, projectors, fibre-opticcommunications, backlight apparatus or other polarisation dependentswitching apparatus.

The component may be used as a polarisation dependent diffractiveelement is shown in FIG. 9. A dual function device is formed from afirst surface relief diffractive structure formed by a substrate 244, anisotropic polymer 234, and a liquid crystal polymer 236 with a surfacerelief diffractive interface 238. A second diffractive element is formedby a second substrate 245, an isotropic polymer 237, and a liquidcrystal polymer 235 with a surface relief diffractive interface 239. Aquarter waveplate 248 is fitted to the output of the device. The devicecan be used for example to image light spots onto two separate surfaces,such as layers of a dual layer optical medium. Light of a firstpolarisation state 240 falls onto the device. The first element has anindex step at the interface 238 for light of this polarisation state,but no index step for light of the second polarisation state 242. Thediffractive interface 238 is arranged to direct light onto a first spot252 on the first surface of the substrate 250. The waveplate 248converts the polarisation state to circular polarisation. The seconddiffractive element images the polarisation 242 to a second spot to asecond spot 254 at a different surface of the substrate 250.Alternatively, the diffractive elements 238,239 could be replaced bylens structures such as Fresnel lenses with the same effect. Theinvention advantageously produces separate focal points for differentpolarisation states. Such an embodiment overcomes the need forbeamsplitters elements to combine the outputs of two focusing systems,and thus is cheaper and simpler to manufacture. Such an element can beused for example in a DVD read/write mechanism and optical positionencoders.

In all of the above embodiments, further layers such as pressuresensitive adhesives and hard coats may be applied to the outer layer ofthe elements subsequent to fabrication of the birefringent layer. Suchlayers may be applied through a coating process.

In a further embodiment, the layer 4 may be formed from a materialcomprising dipoles that can be aligned to provide a liquid crystalaligning function. As shown in FIG. 10 a, the material of the layer 4may have a surface relief structure formed on its upper surface, and thesurface rubbed to form an alignment direction, aligned with thelenticular axis. Advantageously, the requirement to form an additionalalignment layer 6 on the layer 4 may be removed, thus reducing cost andtemperature of processing that would be required for alignment layerbaking. The material of the layer 4 may be birefringent with anamorphous or partially aligned dipole structure. Alternatively, thematerial of the layer 4 may have uniformly aligned dipoles in which oneof the refractive indices of the material of the layer 4 is matched toat least one of the indicia of the birefringent material 12 with a fastaxis alignment such that for one polarisation state, the two materialshave substantially the same refractive index. For the orthogonalpolarisation state, the materials have different refractive indexdetermined by the birefringence of each of the materials.

In one illustrative example, the material of the layer 4 may be athermoplastic material, which is formed using a stretching process suchthat a higher refractive index is seen for polarised light parallel tothe stretch direction 5. Alternatively, the higher refractive index maybe orthogonal to the stretch direction 5. A surface relief structure isthen formed on to one of its surfaces, for example by embossing. Thishas a lenticular structure with geometric axis direction 3 orientedorthogonally to the stretch direction 5 of the film. The material isrubbed parallel to the lenticular axis direction 3 to realign thedipoles at the surface of the layer. A liquid crystal layer is appliedto the surface of the material of the layer 4 using the method of thepresent invention, without the requirement for further alignment layers,forming the structure as shown in FIG. 10 b. The refractive index of thematerial of the layer 4 in a direction parallel to the stretch direction5 of the film (orthogonal to the lens axis direction 3) is index matchedwith the liquid crystal 12, while the refractive index orthogonal to thestretch direction 5 (parallel to the lens axis direction 3) forms arefractive index step with the liquid crystal 12. Light of apolarisation parallel to the stretch direction sees substantially nooptical function, while light polarised orthogonally sees an index step.In this way, advantageously the optical power of the lens can beincreased.

The material of the layer 4 may be a birefringent thermoplastic, such asPET or polycarbonate, a liquid crystal or other birefringent material.Typically, the birefringence of the material of the layer 4 will besignificantly lower than the birefringence of the liquid crystal 12. Thedirections 3 and 5 may alternatively be parallel.

Such lenses may conveniently be formed in a web process. In all of theembodiments of the present invention, the substrate 2 may bebirefringent. In the case in which the substrate is between a polariserand the lens structure 4, 12 then at least one of the indicia of thesubstrate 2 may be aligned parallel or orthogonal to the direction ofthe alignment of the liquid crystal 12 at the surface of the material 4.In the case where the substrate 2 is between the lens 4, 12 and theoutput of the display and in which there is no output polariser, thenthe substrate may have an arbitrary alignment. The structures of thepresent invention may be conveniently formed using a web or roll-to-rollprocess. This may be used to reduced device cost and increase the sizeof fabricated devices.

In a further application of the elements of the invention, it may bedesirable to increase the optical power of the elements that can be madewith available materials. For example, it may be desirable to reduce thefocal length of lenses of the invention. Available isotropic materialstypically have refractive indices in the region 1.45-1.6. Availableliquid crystal materials typically have birefringence in the region0.06-0.3. Curable liquid crystal materials typically have birefringencein the region 0.12-0.2. The optical power of a surface can be increasedby reducing the radius of curvature of the surface. However, if theradius of curvature becomes too small, the surface has high tilt andhigh sag, such that the liquid crystal layer thickness increases,aberration effects increase and the total internal reflection effectsmay become noticeable. Further lenses of equivalent optical power madefrom two curved surfaces with a lower index step typically have lessFresnel reflections than one surface with a high birefringence.

Desirably, the optical performance of the lens can be enhanced by thecombination of two refractive surfaces such as shown in FIG. 11 a. Firstand second lens arrays made in the manner of the current inventioncomprises substrates 300, 302, isotropic materials 304, 306,birefringent materials 308, 310 and an adhesive or optical couplingmaterial 312. Alignment between the two surfaces may be achieved bymeans of active optical alignment using microscopes and translationstages prior to attachment for example. FIG. 11 b shows a furtherpassive alignment mechanism. During coating, a region of the isotropicsurface may be uncoated to expose registration surfaces 316, 318. Analignment device 314 is inserted to provide lateral alignment betweenthe two surfaces. Such an alignment device may for example be a ball orfibre.

In another application of the elements of the present invention, adirectional display may be configured by attachment of a birefringentlens array of the present invention to the front of a polarised outputdisplay such as a Liquid Crystal Display with pixel plane 319 as shownin FIG. 12. For example, the output polariser 320 of the display mayhave an electric vector transmission direction 322. The alignmentdirection 326 on the plane surface 324 of the lens array may be parallelto the direction 322, while the alignment direction 330 on the lensarray surface 328 may be parallel to the lens optical axis direction331.

Such an element provides a high quality lens array with low surfacevisibility using known solid materials which are low cost andstraightforward to handle without requiring for example sealing ofliquid materials. Advantageously, the lenses are aligned with thepolarisation direction of the panel, such that incident light of onepolarisation state (which is index matched between the isotropic andbirefringent materials) is not reflected at the lens interface, whichhalves the visibility of the lens array surface. In the orthogonalpolarisation state, the Fresnel reflectivity and total internalreflection artifacts are substantially reduced compared to a lenssurface in air. The front surface of the optical element can further beanti-reflection and hard-coat coated so as to reduce specularreflections from the display in air and to increase display durability.The lenses may have a tilted optical axis direction 331 compared to thedirection of the pixel columns of the display 318.

Compared to prior art displays, such a display has low levels of lensvisibility in brightly lit ambient environments, thus producing higherquality 3D images. The visibility of the display surface is reduced, sothat the 3D images may extend forward from the display surface (crosseddisparity condition) without visual conflict from the display surface.Such lenses advantageously provide bright 3D images with large amountsof image depth.

In a further embodiment of the invention, the cured liquid crystalmaterial 12 may further be bonded to a carrier substrate 340 by means ofan adhesive layer 342 and the cured polymer material 12 may then bedetached from the layers 4 and 6 so that surface relief birefringentlenses in air are formed as shown in FIG. 13 a. The cured polymer maythen be detached from the material of the layer 4, so that surfacerelief birefringent lenses in air are formed. Such lenses may be usedtogether with a display device 346 using a further intermediate material344 with refractive index matched to one of the indicia of thebirefringent material, as shown in FIG. 13 b. Conveniently, the carriersubstrate 2 and material 4 used during coating may be different from thesubstrate 340 and index matching material 344 used during operation ofthe device. For example the material 4 may be tuned to provide analignment function and a release function without requiring tuning ofrefractive index, while the material 344 may be tuned in refractiveindex and for adhesion. Thus the performance of the device can beoptimised.

1. A non-switching autostereoscopic display apparatus comprising: adisplay apparatus with a polariser disposed between a pixel plane andthe output of the display, and a birefringent liquid crystal component,wherein the birefringent liquid crystal component is arranged to providesubstantially a lens function for polarised light parallel to the outputpolarisation of the display and substantially no lens function for lightof an orthogonal polarisation state.
 2. A display apparatus according toclaim 1, wherein the birefringent liquid crystal component is formed by:forming (a) a layer of first material having an outer surface which isshaped with a surface relief structure and is provided with a liquidcrystal alignment property, and (b) a flexible sheet having an outersurface provided with a liquid crystal alignment property; applying acurable birefringent liquid crystal material to one or both of the outersurface of the layer of first material and the outer surface of theflexible sheet; applying the flexible sheet over the layer of firstmaterial with the outer surfaces of the layer of first material and theflexible sheet facing one another with the curable birefringent liquidcrystal material therebetween, thereby to form a liquid crystal cell;curing the curable birefringent liquid crystal material; and removingthe flexible sheet from the liquid crystal cell, thereby to form abirefringent liquid crystal component.
 3. A display apparatus accordingto claim 1, wherein the birefringent optical element has a twist betweenthe alignment directions of the liquid crystal at the surface reliefstructure and the plane substrate.